In underwater seismic exploration, energy sources are used to create compressional waves that propagate through the water and into underwater land formations. Interfaces between different strata in the land formation reflect the compressional waves back toward a receiver. The receivers commonly used in such underwater seismic explorations are hydrophones which convert the compressional or pressure waves into electrical signals. The signals in turn are processed to form a structural image of the subterranean formation.
Prior art of general interest include the following:
U.S. Pat. No. 3,371,310 discloses a system comprised of vertically disposed components including a seismic generator at or near the earth's surface, a plurality of down-reflecting interfaces below the generator, and a directional seismic-wave detector sensitive to vertical wave travel below the down-reflecting surfaces. Vertically traveling waves received by the detector are phonographically recorded, and through use of the directional properties of the detector are separated into up-traveling and down-traveling seismic waves. The down-traveling seismic waves are modified by omitting the direct waves, and thereafter are mathematically convolved with an up-traveling wave. The resulting convolution function contains events at the times of multiple reflections, but not at the times of primary reflections, and therefore is useful in substantially eliminating, multiple reflections from the seismic wave traces. No calculation of up- and down-going wavefields is made. Further, no determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint.
U.S. Pat. No. 4,809,239 discloses a method of separating seismic waves into compression and shear plane waves, and deriving the velocity and direction of propagation of the plane waves to provide a best fit of the plane wave model to the seismic measurements. More particularly, transducers are placed vertically in a borehole to form an x-z plane intersecting a formation. When an acoustic source is excited at the surface of the formation, compression and shear waves are detected at the transducers to obtain a plural axis measurement of formation displacement. At each transducer depth, compression and shear wave velocities and angles of incidence relative to the x-z axis are estimated, and the phase shifts of the compression and shear waves at each transducer are calculated as a function of the estimates. In addition, a predetermined error function is calculated as a function of the phase shifts, and the estimated velocities and angles of incidence associated with the least predetermined error within a predetermined tolerance are selected. No calculation of up- and down-going wavefields is made, and no determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint. Further, the patent deals with the separation of pressure and shear waves, rather than the separation of pressure waves into up-going and down-going waves.
U.S. Pat. No. 4,935,903 discloses a method and system comprising a seismic energy source, a detector (vertically spaced hydrophones or pressure sensors), a wavefield separator for separating seismic waves into upward traveling and downward traveling components, a time shift element for aligning the components, and a signal enhancement element for forming an improved seismic reflection signal which is subjected to NMO correction. No determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint. Further, rather than achieve wavefield separation through an accurate determination of travel time separation, the patent discloses a less accurate method of using both water pressure and water particle velocity measurements to separate a wavefield into up-going and down-going components.
U.S. Pat. No. 5,005,159 discloses a method of continuity logging in which a seismic source is placed in a first vertical borehole in near proximity to a lithographic layer, and vertically spaced geophones are placed in a second vertical borehole to develop a difference signal. The differential signal is used to reduce the amplitude of primary or secondary waves, while enhancing the amplitude of guided waves. If the guided waves have appreciable amplitude, then continuity is deemed to be present in the lithographic layer. No calculation of up- and down-going wavefields is made. Further, no determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint.
U.S. Pat. No. 5,191,557 discloses a seismic signal processing method and system, which includes a drilling rig seismic source, a drilling rig reference sensor, and plural horizontally spaced surface geophones for receiving both direct and reflected seismic signals. A reference signal is cross-correlated with signals received by geophones at a distance from the borehole. As a result of the cross-correlation, drill bit generated energy can be distinguished from interference and the travelpath to a reflector may be reduced by as much as a factor of 2. Wavefield separation into direct and reflected signals occurs by exploiting moveout differences. No determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint.
U.S. Pat. No. 5,384,752 discloses a method for correcting a seismic pulse waveform for use in the deconvolution of reflected pulse data. The source pulse waveform is measured by a hydrophone positioned vertically beneath the acoustic source. A phase error is calculated based upon the depth of the source and the depth of the hydrophone, and subtracted from the phase spectrum of the measured waveform. The depth of the hydrophone is determined by the travel time to the hydrophone. The resulting waveform provides a representation of a far field waveform. No determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint.
U.S. Pat. No. 5,790,473 discloses a method and system, which include multiple seismic vibratory sources that are energized in accordance with a predetermined pattern. Seismic signals reflected by interfaces separating formations produce motions which are detected by a seismic receiver or geophone located within a borehole. The geophone is moved to different locations in the borehole to measure the motions, which are related to the actual vibrator applied force by a transfer function of a minimum phase. The pattern is used to separate the received data according to the source used to generate the detector motion, and to isolate down-traveling waves for each vibratory source. The separated received data then is correlated by using the down-traveling wave data to produce a seismogram. No calculation of up- and down-going wavefields is made. Further, no determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint.
U.S. Pat. No. 5,742,560 discloses a method of imaging a geological feature. Travel times between two different acquisition datums are determined from measurements or an assumed velocity field. The travel times are used in a wave equation extrapolation of data from actual source and receiver positions to simulated source and receiver positions. Standard imaging techniques thereafter are used to process the data from the simulated source and receiver positions. Vertically displaced sources and receivers are shown for cross-borehole surveying. No calculation of up- and down-going wavefields is made. Further, no determination is made of travel time separation between vertically separated hydrophones, travel time between the surface of the water and a mid-point between the hydrophones, or depth of the midpoint.
U.S. Pat. No.5,581,514 discloses a seismic profile system and method using two vertical strings of vertically spaced sensors, with the vertical strings being horizontally spaced apart. The time delay for a seismic wavefield to travel from a lower sensor to an upper sensor of a sensor pair is calculated by using a cross-correlation technique. Based upon the result, the interval velocity of the medium in which the upper and lower sensors are deployed is statistically estimated. Discrimination between up-going and down-going seismic waves is accomplished. A deconvolution operator then is calculated from the downgoing wavefield, and applied to the up-going wavefield. Vertical spacing of sensors is predetermined, and not calculated. Further, depth of the receiver assembly is not considered.
U.S. Pat. No. 4,794,573 discloses a method for separating upgoing and downgoing seismic waves in a vertical seismic profile. A plurality of vertically spaced geophones are suspended by cable and in contact with the wall of the well borehole. Two geophone signals are processed at a time. The two geophone signals are summed, and the difference between the two signals is time integrated to preserve phase. The integrated difference then is amplitude scale corrected to approximate arrival times for a detector depth midway between the detector pair, and either added to the sum of the two geophone signals to enhance down-going seismic signals or subtracted from the sum to enhance up-going seismic signals. The patent does not determine receiver depth, and does not teach a division of the frequency spectra of the up-going and down-going wavefields in estimating separation distance between the geophones. Rather, the patent assumes that the zero lag cross-correlation .phi..sub.0 (u,d) of the upgoing and downgoing wavefields is zero, which assumption gives rise to substantial error in the determination of the time delay of a pressure wave traveling between a detector pair mid-point and the points of acquisition.